The present invention relates to a sewing machine having a cutting device for producing a cut such as a buttonhole in a workpiece. This type of cutting device generally comprises a mover fastened to the arm head of the sewing machine, which moves a rod vertically against the action of a compression spring. A block is attached to the lower end of the rod. A knife and a cutting plate cooperate to perform the cutting, either the cutting plate or the knife being fastened via a holder to the block
A buttonhole cutting device of this general type is known from German Utility Model No. 76 34 151. In accordance therewith, a stationary knife cooperates during the cutting process with a cutting plate which is moveable vertically upward and downward and consists of visco-elastic plastic. The knife is arranged adjustably and replaceably below a throat plate in a block which is mounted in the machine housing The cutting plate which acts as abutment is received by a holder which is fastened in displaceable position on a block. The latter is fastened on a vertically moveable rod which is part of a mover which is mounted on an arm head of a sewing machine. In order to effect a dependable cut, the cutting plate is moved in the direction towards the knife in such a manner that their facing active cutting edges temporarily contact each other over the full length in the horizontal plane and then--at the end of the cutting process--one cutting edge of the knife penetrates by a predetermined amount of about 0.3 to 0.5 mm into the cutting plate. In order to satisfy the requirements which have just been described, the rod which receives the block is brought into its low position. In this position, the cutting plate is so directed relative to the horizontally extending cutting edge of the knife as to assure a flat full-surface resting of an upper edge of the cutting plate with respect to the cutting edge of the knife. Thereupon the rod and, with it, the cutting plate, is brought into its upper position and the resulting distance between the upper edge of the cutting plate and the cutting edge of the knife is determined. Based on this distance measurement, the cutting plate which is aligned towards the knife is subsequently displaced by the aforementioned amount in the direction towards the knife so that the cutting edge of the knife can penetrate into the cutting plate by the aforementioned amount when the cutting plate again descends.
By the subsequent displacement of the cutting plate, its previously assumed flat alignment with respect to the cutting edge of the knife is again lost, since the two adjustment processes just described, which are to be effected independently of each other, mutually affect each other. Thus the known cutting device has the disadvantage that the prerequisites required for a dependable cut--namely temporary flat contact of the two active cutting edges, as well as the maintaining of a prescribed depth of penetration of the cutting edge of the knife into the cutting plate--can be satisfied only by a large number of time-consuming adjustment processes.